The present invention relates to a mobile communication system which utilizes a radio scheme to secure communication lines for mobile-terminals moving in a wide area.
Typically, a cellular mobile telephone system has hitherto been utilized as this type of mobile communication system.
In the cellular mobile telephone system having a construction as schematically shown in FIG. 1, a terminating call from a public service telephone network (hereinafter abbreviated as PSTN) 101 to a mobile load or terminal (hereinafter abbreviated as ML) 102 once enters a mobile telephone switching office (hereinafter abbreviated as HTSO) 103 and the HTSO 103 causes a base station (hereinafter abbreviated as BS) 104 serving under the MTSO to call the ML 102 of interest. When this ML 102 responds, the MTSO 103 allots a radio channel between the ML 102 and the BS 104 covering the cellular zone in which the ml 102 exists to establish a communication line. In the case of an originating call from the ML 102, the MTSO 103 similarly allots a radio channel between the ML 102 and the BS 104.
Generally, the BS 104 has previously been assigned a predetermined number of radio channels and a channel for communication with the ML 102 is selected from unused radio channels in advance of communication with the ML 102.
In the cellular car telephone system, the service area is divided into small zones each having a BS and the same frequency can be used repetitively for BS's 104 which are so remote from each other that no interference disturbance takes place therebetween, to cover a wide area by taking advantage of highly efficient utilization of frequency. Therefore, this type of system is widely employed in many countries.
Especially, miniaturization of mobile loads or terminals has recently been developed remarkably to promote the number of portable terminals utilizing the cellular mobile telephone system and concurrent cost reduction of terminals drastically increases the number of users.
To comply with the rapidly increasing demand, division of the radio channel interval on the frequency axis, digitization ordering about highly efficient modulation techniques and highly efficient voice coding techniques and the like are adopted with a view of increasing the subscriber's capacity.
In essentiality, however, the cellular mobile telephone system faces the following problems and therefore expansion of the service area and subscriber's capacity cannot proceed infinitely but is limited to a given extent.
1-1 Basically, line design is conducted on the presupposition of outdoor services and, in buildings and underground district, there occurs an area in which services are difficult to offer.
1-2 For increasing the subscriber's capacity in a certain area, division of a zone of interest is the most effective approach (generally called "cell split"). In this case, there needs a great deal of economical and labor burden as exemplified by line design consideration for establishment of new neighboring base stations, change of assignment of radio channels to base stations and investigation of service area.
1-3 When the radius of a zone subject to cell split is reduced in the extreme, the probability that a moving mobile terminal deviates from the zone increases to increase communication processing traffic within the MTSO such as hand-over and the overall performance of the system falls off. Therefore, the zone radius is considered to be limited to several of hundreds meters.
Especially, widespread use of portable terminals has recently grown conspicuously and the communication base capable of offering personal communication has been demanded. Only the cellular mobile telephone system is however insufficient in providing such capability in to pass an area in which the density of population of pedestrians is high is desired to be covered while supporting loads or terminals moving at high speeds in a wide area.